Field of the Invention
The present invention relates to an audio output circuit of a condenser microphone, and especially relates to improvement of an audio output circuit that outputs an audio signal in a balanced line.
Description of the Related Art
A condenser microphone has a diaphragm and a fixed electrode disposed opposite and has an extremely high output impedance, and thus requires an impedance conversion circuit that converts the output impedance to low impedance.
In the impedance conversion circuit, a field effect transistor (FET) or a vacuum tube is used. However, many of the condenser microphones currently use the former impedance conversion circuit using an FET.
Further, an audio signal from the impedance conversion circuit is converted into a balanced output signal, and is supplied to a microphone amplifier such as a mixer through a shielded balanced cable. Further, a known phantom power supply is equipped at the microphone amplifier side, and an operating power is supplied to a condenser microphone side through the shielded balanced cable.
That is, the condenser microphones using the shielded balanced cable is insusceptible to an influence of external noises by transmitting the audio signal in a balanced line and allows to obtain the operating power for an audio output circuit including the impedance conversion circuit from the microphone amplifier side at the same time.
Incidentally, a gooseneck-type microphone is known as a microphone that operates the audio output circuit including the impedance conversion circuit and controls lighting of a light-emitting device mounted at a microphone main body side using the operating power from the phantom power supply.
This gooseneck-type microphone is favorably used as a conference microphone installed on each of speech tables of conference rooms or tables of conference attendees. The gooseneck-type microphone includes a stand arm with a long neck including a flexible pipe that enables easy adjustment of the angle and height, and a microphone main body that houses a condenser microphone unit is attached to a distal portion of the stand arm.
The gooseneck-type microphone installed in conference rooms or the like includes the light-emitting device at the microphone main body side to realize smooth progress of conferences, and a chairperson of a conference or an operator remotely operates the gooseneck-type microphone, so that light-on (light-emitting) and light-off (turning-off) operations are performed. Accordingly, a speaker whose light-emitting device is lighted is prompted to speak, and the conference can smoothly proceed.
As the light-emitting device, an electric lamp or an LED has been conventionally used, and many of these sorts of microphones currently use an LED having a small power consumption and favorable visibility as the light-emitting device.
FIG. 1 illustrates an example of a gooseneck-type microphone including a light-emitting device. A gooseneck-type microphone 1 illustrated in FIG. 1 is composed of abase unit 3 including, for example, a three-pin type output connector 2 detachably mounted to a socket which is attached on a table surface of a conference table or the like, a stand arm 4 attached to the base unit 3, and a microphone main body 5 attached to an upper end portion of the stand arm 4.
The stand arm 4 is formed of a central relay pipe 4a and flexible pipes 4b and 4c attached to upper and lower both end portions of the relay pipe 4a. 
Further, a condenser microphone unit 6 and a circuit board 7 on which an audio signal output circuit including an impedance conversion circuit is mounted are housed in the microphone main body 5 attached to the upper end portion of the stand arm 4. Further, alight guide body 8 formed of a semitransparent resin material is further attached along a peripheral side surface near a lower end portion of the microphone main body 5.
Although not illustrated in FIG. 1, an LED as the light-emitting device is housed to face the light guide body 8 in the microphone main body 5.
FIG. 2 illustrates an example of an audio output circuit in a conventional condenser microphone that lights an LED as a light-emitting device using electric power supplied to the audio output circuit of the condenser microphone from the above-described phantom power supply.
Note that the circuit configuration illustrated in FIG. 2 is the same as an embodiment according to present invention illustrated in FIG. 3 described below, except for a connection of source resistors that configure first and second impedance conversion circuits. Therefore, in FIGS. 2 and 3, portions serving the same function are denoted with the same reference symbol, and a detailed connection configuration will be described below based on FIG. 3.
The audio output circuit illustrated in FIG. 2 includes first and second impedance conversion circuits using source follower circuits respectively composed of two N-channel type FETs Q1 and Q2, and first and second current amplifier circuits (output circuits) using emitter follower circuits composed of two PNP-type transistors Q3 and Q4 directly connected to the impedance conversion circuits, respectively.
An operating current is supplied from the phantom power supply mounted to a microphone amplifier side such as a mixer (not illustrated) to the FET Q1 comprising the first impedance conversion circuit and the transistor Q3 comprising the first current amplifier circuit through a terminal pin PIN2 assigned as a hot-side output terminal of an output connector.
Further, an operating current is supplied from the phantom power supply to the FET Q2 comprising the second impedance conversion circuit and the transistor Q4 comprising the second current amplifier circuit through a terminal pin PIN3 assigned as a cold-side output terminal of the output connector.
A condenser microphone unit 6 is connected to a gate of the FET Q1, and a signal from the condenser microphone unit 6 is outputted with a converted impedance to a source resistor R1 of the FET Q1. A source output thereof is inputted to a base of the transistor Q3, and an emitter output which is current-amplified by the transistor Q3 is outputted to the terminal pin PIN2 as a hot-side audio signal.
Further, the emitter output of the transistor Q3 is supplied to a gate of the FET Q2 through a low pass filter made of a resistor R3 and a condenser C1, and a DC-cut condenser C2. A source output with converted impedance output to a source resistor R2 of the FET Q2 is inputted to abase of the transistor Q4, and an emitter output with a current amplified by the transistor Q4 is outputted to the terminal pin PIN3 as a cold-side audio signal.
With the above-described configuration, the audio signal from the condenser microphone unit 6 is outputted in a balanced line to the terminal pins PIN2 and PIN3 of the output connector as the hot-side and cold-side audio signals, respectively.
The condenser microphone having the above-described circuit configuration is disclosed, especially in FIG. 3, in JP 2015-97312 A.
In the circuit configuration illustrated in FIG. 2, four LEDs connected in series, with the reference symbol LE1, are mounted as the light-emitting device.
Further, current-regulated diodes CR1 and CR2 are included whose anodes are connected respectively to the terminal pins PIN2 and PIN 3 of the output connector. Cathodes of the current-regulated diodes CR1 and CR2 are commonly connected, and the LEDs are connected in series to a common connection point of the cathodes, and a drive current is supplied from the current-regulated diodes.
Note that, in the circuit example illustrated in FIG. 2, a cathode of the LEDs connected in series with the reference symbol LE1 is connected to the terminal pin PIN1 of the output connector. That is, in this circuit example, the terminal pin PIN1 is used for light on/off control of the LEDs, and the audio output circuit illustrated in FIG. 2 is ground-connected with the microphone amplifier side using a frame ground terminal SI of the output connector.
Therefore, as illustrated in FIG. 2, a switch SW is connected to between the terminal pin PIN1 and the frame ground terminal SI of the output connector, to thereby remotely operate the light on/off operations of the LEDs illustrated by the reference symbol LE1 and connected in series.